Admixtures are materials added to concrete prior or during the mixing process to modify one or more of the properties of concrete in its plastic or hardened state. They can be natural or synthetic and may be added in small quantities to produce a specific effect.
The reducing action of a water-reducing admixture can result in reduced water-cement ratios which, as a rule, allow concrete to be produced with higher strength levels and lower density than untreated concrete. Commonly, concrete containing a water-reducing admixture will require about 5 to 10 percent less water than concrete based on the same mix design and aggregates.
The retarding action of a retarding admixture is often used in hot weather to increase the set time of concrete. This admixture can be added either during batching or at the jobsite. The effects of a retarding admixture may be diminished by prolonged haul times, so it is important to use it only when the haul schedule can be met.
Another type of admixture, also known as high-range water reducers or MRWRs, are capable of reducing the water content of concrete by up to 30%. This provides a more fluid and workable mix that can be used in applications where vibration consolidation cannot be achieved.
Natural pozzolans, such as volcanic ash and fly ash, can be used to improve the properties of concrete. Pozzolans are cementitious materials and can be used in place of Portland cement or blended with it. They affect the hardened concrete through hydraulic or pozzolanic activity, increasing the strength of concrete and reducing its early compressive strength development.